Should we allow optional secret provider registrations?

[stijnmoreels]

@fgheysels :

Is your feature request related to a problem? Please describe.
As proposed here, @fgheysels makes the point that we could include optional registrations for the secret providers in the secret store (like provided in the configuration providers).

Describe the solution you'd like
What needs to be figured out is:

Should we provide the option to register all secret providers as optional registrations?
If so, should we ignore/log the exceptions when the secret is not found instead of blocking?
If so, should we allow invalid/incorrect setup of the secret provider with possible unauthorized exceptions to be ignored when the secret provider was registered as optional?
Additional context
#171 (comment)
#200 (original issue)